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PA N @FHQE ROBERT H. nous AND rams a. swim, or PHILADELPHIA, PENNSYLVANIA.

tans:

Specification of letterslatent.

Patented Jan, 319, 1915.

91mm. meeina 20, 1914. sem no. eaaaar.

To-all' whomz't camera:

Be it known that we, .ROBERT H. Herman and LEWIS J. STERN, citizens of the United States, residing in the city and county-of 'Philadelphia, Pennsylvania, have invented new and useful Improvements in Lenses, of which the following is a description.

The object of this invention is a lens so constructed as to transmit a convergentlor divergent beam, and take in a very large solid angle of light'from a point source.

For any given value of the index of refraction the solid angle taken in by a lens constructed with spherical surfaces is limp the principal foci, more than four times the light will be pf 'ojected in a parallel beam by our lens than by the spherical lens. This quantity of light may be practically doubled by placing a spherical mirror back of the source with its center of curvature at the principal focus of our lens.

The/surfaces of our lens are not spherical. They are however, surfaces of revolution, the generating curves of which are definitely defined in terms of an auxiliary curve'used as a directrix. The characteristic of this auxiliary curve is a constantly decreasing curvature. The parabola may be taken as a type of these curves and is used in the following description.

References are made to the diagram accompanying and forming part of this specification.

The cross-hatched portion of the diagram shows a section of our lens taken through the axis of revolution. The other portion shows the construction of the generating curves.

P'OP is a parabola having its focus at F and its directrix DD. The curve TT is constructed with the normals to the parabola and the radii vectors from F so that its normals make angles with the radii vectors and the normals to the parabola having the index of refraction as the ratio of their sines. Therefore light diverging from F incident upon the surface of revolution TT' is refracted along the normals to the auxiliary: parabola P OP The curve ZZ is constructed with the normals to the parabola and the normals to the directrix of the parabola so that its normals make angles with the normals to the parabola and the normals to-the directrix of the parabola having the reciprocal of the index of refraction at the ratio of their sines. Therefore light along the. normals to the auxiliary parabola POP' incident upon the surface of revolution ZZ will be refracted alon the normals to the directrix of the auxlliary parabola P OP or light diverging from F along the radii vectors strikes the surface TT' is refracted along the normals to the auxiliary parabola POP and emerges as a parallel beam. The practical construction of these curves is as follows: Commencing at any point L draw LR, a nor mal to the auxiliary parabola POP' and lay off LR. Construct LF and lay off LR such that the ratio of LB to LB is the index of refraction. Construct .NN through L parallel to RR. The perpendicular to N N at L gives the direction of the surface at L, such that light from F to L is refracted along R L At a short distance from L along the perpendicular to NN re- HH at L gives the direction of the surface at L, such that light along RL is refracted along RL'. At a short distance from L along the perpendicular to-HH' repeat the construction, and so on; in this way the curve ZZ was constructed point by point. While it is diflicult to grind a lens having these surfaces, the construction of a pressed lens of this design is reduced to the question of the construction of the mold.

It is not our purpose to restrict ourselves to the use of the parabola as a directrix, because as stated above other curves are suited to the purpose.

What we claim as new and desire to secure by Letters Patent of the United States A lens having surfaces of revolution, the parallel beam, substantially as set forth in generating curves of which are defined in the above specification. terms of the index .of refraction and the ROBERT HOUGH normals to an auxiliary curve of decreasing curvature taken as a directrix such that'the LEWIS STERN light diverging from the principal focus is Witnesses: refracted along the normals to the auxil- DAVID T. Nnvm, iary curve and emerges from the lens in a S. Knmnn KENT. 

